“Every intelligent individual wants to know what makes him tick, and yet is at once fascinated and frustrated by the fact that oneself is the most difficult of all things to know.”

~ Alan Watts**The Card Paradox**

Invented by the British logician Philip Jourdain in the early 1900s, the Card Paradox is a simple variation of what is known as a “liar paradox,” in which assigning truth values to statements that purport to be either true or false produces a contradiction.

Imagine you’re holding a postcard in your hand, on one side of which is written, “The statement on the other side of this card is true.” We’ll call that Side-A. Turn the card over, and the opposite side reads, “The statement on the other side of this card is false” (Side-B). Trying to assign any truth to either Statement Side-A or Side-B, however, leads to a paradox: if Side-A is true then Side-B must be as well, but for Side-B to be true, Side-A has to be false. Oppositely, if Side-A is false then Side-B must be false too, which must ultimately make Side-A true, and yet the paradox!

So which side is True and which side is False?

**Epimenides paradox**

Thomas Fowler (1869) states the paradox as follows: "Epimenides the Cretan says, 'that all the Cretans are liars,' but Epimenides is himself a Cretan; therefore he is himself a liar. But if he be a liar, what he says is untrue, and consequently the Cretans are veracious; but Epimenides is a Cretan, and therefore what he says is true; saying the Cretans are liars, Epimenides is himself a liar, and what he says is untrue. Thus we may go on alternately proving that Epimenides and the Cretans are truthful and untruthful."

So What is Cretans, Liars or Truthful?

**The Bootstrap Paradox**

Imagine that a time traveller buys a copy of Hamlet from a bookstore, travels back in time to Elizabethan London, and hands the book to Shakespeare, who then copies it out and claims it as his own work. Over the centuries that follow, Hamlet is reprinted and reproduced countless times until finally a copy of it ends up back in the same original bookstore, where the time traveller finds it, buys it, and takes it back to Shakespeare.

Who, then, wrote Hamlet?